Holography provides a promising technique for storing digital data at high density with rapid access times. In a page-based holographic system, data is stored by passing a monochromatic signal beam through a spatial light modulator having portions of varying degrees of transparency corresponding to digits of information. The signal beam then strikes a holographic storage medium. A reference beam is also incident upon the storage medium and interferes with the signal beam. In the regions of constructive interference between the two beams, the index of refraction of the storage medium is changed; a hologram representing the page of data displayed on the spatial light modulator is thereby stored.
To read the data stored in the medium, the reference beam, but no signal beam, is incident upon the medium. The regions of altered index of refraction cause a portion of the reference beam to diffract and emerge from the storage medium as a reconstructed signal beam, or an image beam. The image beam carries a holographic image of the page of stored data, and is detected by a pixellated detector, such as a CCD camera. An information pattern is thereby projected upon the detector.
Reference marks of various kinds are often stored with the data in the holographic medium to aid in the reading of the data. Redfield et al., in U.S. Pat. No. 5,519,517, provide fiducial marks in the storage medium, and store the data at known positions with respect to the fiducial marks. The marks are then used for aligning the detector with the data. Hays et al., in U.S. Pat. No. 5,777,760, also store fiducial marks that indicate not only the location of the stored data, but also the relative angle between the reference beam and a surface of the storage medium. This angle needs to be determined, for example, when several pages of data are stored in the same volume of storage medium using angle multiplexing techniques.
Abe at al., in U.S. Pat. No. 4,149,269 describe fiducial marks used to identify information in a page, as well as to set the quantization threshold for converting the detected intensities of the image beam to binary data. Richardson, in European Patent Application EP 0 851 319 A1, also describes fiducial marks, or "test signals," for setting the quantization threshold.
One difficulty with holographic data storage is that the detector pixels are often misaligned with the data pixels of the information pattern detected. One method for addressing this problem is given by Visel et al. in U.S. Pat. No. 5,511,058. Visel et al. process the detected signals to compensate for such a misalignment.
Another problem with holographic data retrieval is distortion of the image received by the detector. The optical components that deliver the signal and image beams generally cause some measure of distortion. The prior art does not address this problem.
Another difficulty with holographic storage is that data can be corrupted. Other storage systems, for example magnetic storage systems, suffer from the same problem. To minimize the impact of the data loss in those systems, typically error correction codes are used, and the bits of each code word are dispersed throughout the magnetic medium. If a small enough region of the magnetic medium is damaged, that region would not typically contain any code word in its entirety, but portions of a number of code words. These lost portions can often be recovered using the error correction code. Methods of dispersing data in magnetic media rely on the one-dimensional nature of a magnetic track.
Ueda, in U.S. Pat. No. 5,265,226, gives a more general method for dispersing data throughout a memory device. What is needed, however, is a method specifically designed for the two-dimensional structure of a page of holographic data Curtis et al., in U.S. Pat. No. 5,812,288, describe a method for dispersing data in a page of holographic data, wherein the symbols of a particular code word occupy different rows and columns of the page. However, Curtis et al. do not provide an optimal algorithm for performing this dispersal.